samples/bpf/xdp_adjust_tail_kern.c

   1 /* SPDX-License-Identifier: GPL-2.0
   2  * Copyright (c) 2018 Facebook
   3  *
   4  * This program is free software; you can redistribute it and/or
   5  * modify it under the terms of version 2 of the GNU General Public
   6  * License as published by the Free Software Foundation.
   7  *
   8  * This program shows how to use bpf_xdp_adjust_tail() by
   9  * generating ICMPv4 "packet to big" (unreachable/ df bit set frag needed
  10  * to be more preice in case of v4)" where receiving packets bigger then
  11  * 600 bytes.
  12  */
  13 #define KBUILD_MODNAME "foo"
  14 #include <uapi/linux/bpf.h>
  15 #include <linux/in.h>
  16 #include <linux/if_ether.h>
  17 #include <linux/if_packet.h>
  18 #include <linux/if_vlan.h>
  19 #include <linux/ip.h>
  20 #include <linux/icmp.h>
  21 #include <bpf/bpf_helpers.h>
  22
  23 #define DEFAULT_TTL 64
  24 #define MAX_PCKT_SIZE 600
  25 #define ICMP_TOOBIG_SIZE 98
  26 #define ICMP_TOOBIG_PAYLOAD_SIZE 92
  27
  28 /* volatile to prevent compiler optimizations */
  29 static volatile __u32 max_pcktsz = MAX_PCKT_SIZE;
  30
  31 struct {
  32         __uint(type, BPF_MAP_TYPE_ARRAY);
  33         __type(key, __u32);
  34         __type(value, __u64);
  35         __uint(max_entries, 1);
  36 } icmpcnt SEC(".maps");
  37
  38 static __always_inline void count_icmp(void)
  39 {
  40         u64 key = 0;
  41         u64 *icmp_count;
  42
  43         icmp_count = bpf_map_lookup_elem(&icmpcnt, &key);
  44         if (icmp_count)
  45                 *icmp_count += 1;
  46 }
  47
  48 static __always_inline void swap_mac(void *data, struct ethhdr *orig_eth)
  49 {
  50         struct ethhdr *eth;
  51
  52         eth = data;
  53         memcpy(eth->h_source, orig_eth->h_dest, ETH_ALEN);
  54         memcpy(eth->h_dest, orig_eth->h_source, ETH_ALEN);
  55         eth->h_proto = orig_eth->h_proto;
  56 }
  57
  58 static __always_inline __u16 csum_fold_helper(__u32 csum)
  59 {
  60         csum = (csum & 0xffff) + (csum >> 16);
  61         return ~((csum & 0xffff) + (csum >> 16));
  62 }
  63
  64 static __always_inline void ipv4_csum(void *data_start, int data_size,
  65                                       __u32 *csum)
  66 {
  67         *csum = bpf_csum_diff(0, 0, data_start, data_size, *csum);
  68         *csum = csum_fold_helper(*csum);
  69 }
  70
  71 static __always_inline int send_icmp4_too_big(struct xdp_md *xdp)
  72 {
  73         int headroom = (int)sizeof(struct iphdr) + (int)sizeof(struct icmphdr);
  74
  75         if (bpf_xdp_adjust_head(xdp, 0 - headroom))
  76                 return XDP_DROP;
  77         void *data = (void *)(long)xdp->data;
  78         void *data_end = (void *)(long)xdp->data_end;
  79
  80         if (data + (ICMP_TOOBIG_SIZE + headroom) > data_end)
  81                 return XDP_DROP;
  82
  83         struct iphdr *iph, *orig_iph;
  84         struct icmphdr *icmp_hdr;
  85         struct ethhdr *orig_eth;
  86         __u32 csum = 0;
  87         __u64 off = 0;
  88
  89         orig_eth = data + headroom;
  90         swap_mac(data, orig_eth);
  91         off += sizeof(struct ethhdr);
  92         iph = data + off;
  93         off += sizeof(struct iphdr);
  94         icmp_hdr = data + off;
  95         off += sizeof(struct icmphdr);
  96         orig_iph = data + off;
  97         icmp_hdr->type = ICMP_DEST_UNREACH;
  98         icmp_hdr->code = ICMP_FRAG_NEEDED;
  99         icmp_hdr->un.frag.mtu = htons(max_pcktsz - sizeof(struct ethhdr));
 100         icmp_hdr->checksum = 0;
 101         ipv4_csum(icmp_hdr, ICMP_TOOBIG_PAYLOAD_SIZE, &csum);
 102         icmp_hdr->checksum = csum;
 103         iph->ttl = DEFAULT_TTL;
 104         iph->daddr = orig_iph->saddr;
 105         iph->saddr = orig_iph->daddr;
 106         iph->version = 4;
 107         iph->ihl = 5;
 108         iph->protocol = IPPROTO_ICMP;
 109         iph->tos = 0;
 110         iph->tot_len = htons(
 111                 ICMP_TOOBIG_SIZE + headroom - sizeof(struct ethhdr));
 112         iph->check = 0;
 113         csum = 0;
 114         ipv4_csum(iph, sizeof(struct iphdr), &csum);
 115         iph->check = csum;
 116         count_icmp();
 117         return XDP_TX;
 118 }
 119
 120
 121 static __always_inline int handle_ipv4(struct xdp_md *xdp)
 122 {
 123         void *data_end = (void *)(long)xdp->data_end;
 124         void *data = (void *)(long)xdp->data;
 125         int pckt_size = data_end - data;
 126         int offset;
 127
 128         if (pckt_size > max(max_pcktsz, ICMP_TOOBIG_SIZE)) {
 129                 offset = pckt_size - ICMP_TOOBIG_SIZE;
 130                 if (bpf_xdp_adjust_tail(xdp, 0 - offset))
 131                         return XDP_PASS;
 132                 return send_icmp4_too_big(xdp);
 133         }
 134         return XDP_PASS;
 135 }
 136
 137 SEC("xdp_icmp")
 138 int _xdp_icmp(struct xdp_md *xdp)
 139 {
 140         void *data_end = (void *)(long)xdp->data_end;
 141         void *data = (void *)(long)xdp->data;
 142         struct ethhdr *eth = data;
 143         __u16 h_proto;
 144
 145         if (eth + 1 > data_end)
 146                 return XDP_DROP;
 147
 148         h_proto = eth->h_proto;
 149
 150         if (h_proto == htons(ETH_P_IP))
 151                 return handle_ipv4(xdp);
 152         else
 153                 return XDP_PASS;
 154 }
 155
 156 char _license[] SEC("license") = "GPL";